1. Field of the Invention
This invention relates to distance measuring systems, and in particular, but not exclusively, to the use of such systems as altimeters, and in particular, but not exclusively, to the use of such altimeters with aircraft such as helicopters and balloons.
More particularly, the distance measuring systems of the present invention are of the type comprising: means for generating a first modulated electrical signal; a transmitting transducer for receiving the first electrical signal and producing a corresponding modulated acoustic signal to be directed towards and reflected by a surface; a receiving transducer arranged to receive the reflected acoustic signal and for producing a corresponding second modulated electrical signal; and processing means for receiving the second electrical signal, or the first and second electrical signals, and correlating the modulations thereof to produce an indication of a time delay thereof from which a distance travelled by the acoustic signal can be determined. From the time delay and the local speed of sound, the distance can, of course, then be determined.
2. Description of the Prior Art
Distance measuring systems of this type are already known, from patent document U.S. Pat. No. 5,173,881 in the context of vehicle proximity sensing, and from patent document WO-A-81/00456 in the context of geological prospecting.
In such systems, it would generally be desirable to used matched transducers for transmitting and receiving. Indeed, due to the reciprocity of typical transducers suitable for this purpose, it is generally desirable for the two transducers to be identical. Furthermore, it would generally seem to be desirable to operate the transducers at their fundamental resonant frequency so that the acoustic signal is as strong as possible.
In accordance with a first aspect the present invention, however, the frequency of the acoustic signal is chosen so that it is not at or near the fundamental resonant frequency of either transducer and so that it satisfies at least one (preferably two or more, and more preferably all) of the following conditions: (i) at or near a higher-order resonant frequency of one or both of the transducers; (ii) at a frequency at which a frequency-dependent directivity index of the transducers is relatively high; (iii) at a frequency at which the ambient acoustic noise level is relatively low; and (iv) at a frequency at which the acoustic reflection loss at the surface is relatively low. As will be appreciated from the following detailed description, this increases the maximum operating range of the system.
A problem which arises in the system of the known type is related to airborne transmission directly between the transmitting transducer and the receiving transducer, rather than via the reflective surface. Typically, direct airborne transmission would produce a greater peak in the correlation than the reflected transmission. In order to deal this problem, the transducers may be spaced further apart. However, the system then becomes less compact. In accordance with a second aspect the present invention, the transmitting and receiving transducers are mounted adjacent each other, and an acoustically absorbent material is mounted between the transmitting and receiving transducers. It has been found that the addition of the acoustically absorbent material not only reduces the direct airborne transmission peak in the correlation, but may also increase and sharpen the reflected transmission peak, and thus it becomes easier to extract the required information in the correlation process.
Even when the second aspect of the invention is employed, it may be the case that the direct airborne transmission peak is greater than the reflected transmission peak in the correlation. In order to deal this problem, in accordance with a third aspect of the present invention, the processing means does not attempt to correlate the first and second electrical signals in respect of a time delay which is less than or generally equal to the time taken for the acoustic signal to travel directly from the transmitting transducer to the receiving transducer. It will be appreciated that this may then reduce the minimum operating range of the system. However, by placing the transducers sufficiently close to each other, for many applications of the system this may not be an issue. It should also be noted that, by employing the second and third aspects of the invention in combination, not only are the effects of direct airborne transmission substantially cancelled out in the correlation, but also the reflected transmission peak may be unexpectedly increased and sharpened, as mentioned above.
For compactness and simplicity, the transmitting and receiving transducers are preferably disposed adjacent each other and provided on a common mounting. However, a problem then arises that mechanical vibrations can be transmitted through the common mounting from the transmitting transducer to the receiving transducer, which may produce sufficient noise to obliterate completely the reflection peak in the correlation. In order to deal with this problem, in accordance with a fourth aspect of the present invention, the transmitting and receiving transducers are mounted adjacent each other, each by a respective mounting system to a common member, and at least one of the mounting systems provides acoustic damping and isolation between the respective transducer and the common member.
In accordance with a fifth aspect the present invention, the first electrical signal is either a carrier wave (such as a sine wave) which is modulated by a random or pseudorandom binary sequence (preferably an M-sequence), or is the binary sequence itself. Pseudo-random binary sequences include sequences which repeat after a finite period. Particularly good results are achieved with repetitive sequences which comprise all binary words that can be made from n binary digits, where n is an integer which can have a value of from one upwards. If the all-zero case is excluded, the sequence is termed a maximal length sequence or M-sequence.
As mentioned above, the distance measuring system may be used with an aircraft, such as a helicopter or balloon, as an altimeter or for obstacle detection.
In particular, in the case of the system of the first aspect of the invention when used with a helicopter, the frequency of the acoustic signal is preferably chosen so that it satisfies at least one of the following conditions: (i) at a frequency at which the acoustic noise level produced by the helicopter below the helicopter is relatively low; and (ii) at a frequency at which the acoustic reflection loss at an interface between air and for example grassland, rock or water is relatively low.
At least one of the transducers may be stationarily mounted with respect to the body of the helicopter. If both transducers are so mounted, the system can be used to measure the altitude of the aircraft above ground or sea level. Additionally or alternatively, at least one of the transducers may be stationarily mounted with respect to a winch-hook, or the like, of the aircraft. If both transducers are so mounted, the system can be used to measure the altitude of the winch load above ground or sea level. If one of the transducers is mounted on the aircraft and the other on the winch-hook, the altitude of the winch hook can still be measured by correlating the modulations of the directly received acoustic signal and the reflected acoustic signal.
In accordance with a sixth aspect of the present invention, there is provided an aircraft having a distance measuring system of the type mentioned previously, and further including means for controlling an altitude-related aspect of the aircraft in dependence upon the indicated time delay. For example, the lift produced by the aircraft may be controlled in dependence upon the indicated time delay so as to control the aircraft altitude above ground or sea level. Additionally or alternatively, the length of a winch line of the aircraft may be controlled in dependence upon the indicated time delay so as to control the height of the load above ground or sea level, regardless (within limits) of the aircraft altitude. With appropriate control in dependence upon the time delay, xe2x80x9csoft landingxe2x80x9d of loads can be readily and reliably achieved.
It is not necessary, in all applications, to employ a transducer to produce a modulated acoustic signal which is directed towards and reflected by the surface. Instead, in the case of an aircraft, it is possible to use noise normally generated by the aircraft, such as by the tips of the rotor of a helicopter, by the aircraft""s engine, by airflow over the aircraft""s fuselage, etc. Accordingly, in accordance with a seventh aspect of the invention, there is provided an aircraft having a distance measuring system comprising: receiving transducer means arranged to receive a modulated acoustic signal generated by the aircraft, both directly and indirectly after reflection by a surface, and to produce a corresponding modulated electrical signal; and processing means for receiving the electrical signal and correlating the modulations thereof to produce an indication of a time delay thereof from which a distance travelled by the reflected acoustic signal can be determined. Preferably, the receiving transducer means comprises a directional transducer which is directed towards the surface so as to enhance the proportion of the sound received from the surface. Also, the receiving tranducer means preferably further comprises a second directional transducer which is directed towards the source of the acoustic signal generated by the aircraft to produce a further electrical signal, the processing means being operable to correlate the modulations of both of the electrical signals.
It should be noted that any of the features of the various aspects of the invention set out above may be may be combined in any combination in a single system.